The present invention relates to a base station equipment of a mobile communication system which is intended to enable a communication with a mobile station with a narrow angle directivity (narrow angle beam) antenna in order to reduce the quantity of interferences.
An adaptive array antenna in a conventional mobile communication base station equipment is constructed by providing a plurality of receivers for each communication channel, adjusting an antenna weight to control the direction of a principal beam in the antenna directivity response, extracting an optimal received signal, and employing the antenna weight which is used for the optimal signal in controlling the direction of a principal beam in the directivity response of a transmitting antenna. However, this practice requires a plurality of transmitters/receivers for each channel for both the transmission and the reception, disadvantageously increasing the scale of the equipment.
To accommodate for this problem, there is proposed a technique as illustrated in FIG. 1 where a beam switcher 12 selectively connects a transmitter 13 to one of a plurality of antennas 11-1 to 11-4 having narrow beam angle directivities 35-1 to 35-4 in mutually different directions through respective duplexers 36-1 to 36-4 while a beam switcher 14 selectively connects a receiver 15 to one of the antennas, thus minimizing the number of transmission/reception network paths. According to this technique, receivers 16-1 to 16-4 are used to measure the signal strength from respective narrow beam antenna 11-1 to 11-4 to allow a beam selection control circuit 17 to switchably control the beam switchers 12, 14 so that transmitter 13 and the receiver 15 may be connected to one of the antennas having the maximum received signal power. With this technique, to scan the arriving direction of a received radiowave, a number of direction finder receivers 16-1 to 16-4 are necessary which is equal to the number of antenna branches, which is four in FIG. 1. When the technique is applied to the mobile communication, which represents a multi-path environment, a difficulty is encountered in establishing an accurate beam switching because of a variation in the signal strength which occurs independently on each antenna branch. (See Tadashi Matsumoto, Seiji Nishioka and David J. Hodder, “Beam-Selection Performance Analysis of a Switched Multibeam Antenna System in Mobile Communications Environments,” IEEE Trans., VT, Vol. 46, No. 1 (February 1997).)
A high resolution signal processing technique such as MUSIC is known in the art to estimate the arriving direction of a radiowave (DOA; Direction of Arrival), but requires a complex treatment including the calculation of a correlation matrix, resulting in a tremendous length of time as the number of antennas increases. (See R. O. Schmidt, “Multiple Emitter Location and Signal Parameter Estimation,” IEEE Trans. AP. Vol-34, No. 3 (March 1986).) The treatment of this technique is even more complicated when plural antenna having different directivities are used. For this reason, it necessitates the provision of an array antenna including antenna elements 18-1 to 18-4 having a common directivity for direction finding purpose, separately from communication antennas, as shown in FIG. 2. Received signals from the antenna elements 18-1 to 18-4 are fed to the receivers 16-1 to 16-4, outputs of which are processed in a circuit 19 according to the MUSIC procedure to determine the direction on which the transmitting mobile station is located, thus controlling the beam switchers 12 and 14.
In the actual operation of the mobile communication, there are users (mobile stations) who move rapidly during the communication intervals and who frequently change the channels on one hand, and there are many users who complete the communications without substantial movements on the other hand. Because the mobile communication base station equipment premises that every user (mobile station) be serviced during a rapid movement thereof, it uses antenna which exhibit a common wide angle directivity response for a plurality of frequency channels and time slots. Thus, when commencing a communication with a particular user (mobile station), the base station equipment is radiating radio waves in directions of its service area such as a sector area, for example, other than the direction on which the user is located, and this represents a wasteful power dissipation. It will thus be seen that the use of antennas which exhibit a common angle directivity response for every frequency channel and time slot is problematic from the standpoints of radio wave environment and power saving. There is then a proposal which uses an array antenna to produce a narrow beam angle directivity response separately for each frequency channel and time slot so that a narrow angle beam be always directed to a user, thus tracking it. The proposed technique is excellent when viewed from above standpoints, but presents problems in that an increased area must be provided for installation of antennas and the equipment must be scaled up. In addition, a complex signal processing system is needed.
A conventional arrangement of base station equipment is shown in FIG. 3. A receiving antenna 111 and a transmitting /receiving antenna 112 are oriented in a common direction and have directivity responses indicated by principal beams 161 and 162, respectively, which are 120° wide. The receiving antenna 111 is directly connected to a combiner and distributor 26 while the transmitting/receiving antenna 112 is connected thereto through a duplexer 36. Each transmitter 13 of transmitter/receiver assemblies 115-1 to 115-L for frequency channels f1s to f1L inclusive of control channels and communication channels is connected to the transmit port of the combiner and distributor 26 while receivers 15-1 and 15-2 are connected to the respective receive port of the combiner and distributor 26 for the antennas 111 and 112, thus allowing the transmission and the reception of the control channel and the communication channel. Communication channel transmitter/receiver assemblies 121-1 to 121M for frequency channels f21 to f2M each include a transmitter 122 which is connected to the transmit port of the combiner and distributor 26 and also each include receivers 123 and 124 which are connected to the respective receive port of the combiner and distributor 26 for the antennas 111 and 112, thus allowing the transmission and the reception of the communication channels. Each of the receivers 15-1 and 15-2 is adapted to diversity reception as is each of the receivers 123 and 124.
Time slots which are utilized by the transmitter/receiver assemblies 115-1 to 115-L are shown in FIG. 4A and time slots which are utilized by the transmitter/receiver assemblies 121-1 to 121-M are shown in FIG. 4B. The beam 162 of the antenna which is used in each transmission has a width of 120°, and this means that a common beam is used for every frequency channel and time slot. A base station controller 126 allocates a channel which is used by either one of the transmitter/receiver assemblies 115-1 to 115-L and 121-1 to 121-M during a particular time slot.
As discussed, the arrangement which employs the direction finding of the mobile station and a result of such scan is used in switching a transmit/receive beam suffers from the accuracy of directional finding, the scale of equipment and the quantity of calculations.
It will also be seen that because a wide angle beam antenna is fixedly assigned to every channel in a conventional base station equioment, this means that the equipment dissipates a wasteful radiation power in directions in its service area (such as a sector, for example) other than the direction on which a desired mobile station is located, contributing to increasing the quantity of interferences with other base stations. It is an object of the invention to provide a mobile communication base station equipment which enables a communication with a mobile station with a narrow angle beam by performing a direction finding of an arriving radio wave at a higher accuracy with a minimum scale of equipment and with a minimum volume of calculations.
It is another object of the invention to provide a mobile communication base station equipment which allows the quantity of interferences caused by radiated power to be reduced as compared with the prior art.
According to a first aspect of the present invention, there are provided a pair of wide angle beam antennas located close to each other for substantially covering a service area which is covered by an entire assembly including a plurality of narrow angle beams. One of the antennas of the pair is connected to a communication receiver while the other antenna is connected to a direction finder receiver. The direction on which a mobile station transmitting a particular received radio wave is located is determined on the basis of phases of received signals from the both receivers. The function of the wide angle beam antenna may be served by one of the plurality of antennas which are used to form the narrow angle beams.
According to a second aspect of the present invention, there are provided a single wide angle beam antenna and a plurality of narrow angle beam antennas which collectively cover a service area of the wide angle beam antenna. A traveling speed of a mobile station and the direction of a narrow angle beam on which the mobile station is located are detected. On the basis of such information, when the traveling speed is high, one of communication channel transmitters/receivers capable of feeding transmitting power is allocated to the wide angle beam antenna while when the traveling speed is low, one of the communication channel transmitters/receivers capable of feeding transmitting power is allocated to the narrow angle beam antenna corresponding to the direction on which the mobile station is located.